1. Field of the Invention
The present invention relates generally to the fabrication of integrated circuits, and more particularly, to a system and method of adhering copper to a diffusion barrier surface.
2. Background of the Related Art
The demand for progressively smaller, less expensive, and more powerful electronic products, in turn, fuels the need for smaller geometry integrated circuits (ICs), and large substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the interconnections between components and dielectric layers be as small as possible. Therefore, research continues into reducing the width of via interconnects and connecting lines. The conductivity of the interconnects is reduced as the surface area of the interconnect is reduced, and the resulting increase in interconnect resistivity has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the IC. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected, or from the joining of conductors having highly different resistivity characteristics.
To meet the need for interconnects and vias having both low resistivity, and the ability to withstand volatile process environments, aluminum and tungsten have been used in the production of integrated circuits. These metals are popular because they are easy to use in a production environment. However, as geometries have become smaller, copper has replaced aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. As a result, the same current can be carried through a copper line having half the width of an aluminum line. The electromigration characteristics of copper are also much superior to those of aluminum. Copper is approximately ten times better than aluminum with respect to electromigration. As a result, a copper line, even one having a much smaller cross-section than an aluminum line, is better able to maintain electrical integrity.
Copper cannot be deposited onto substrates using the conventional processes for the deposition of other metals like aluminum when the geometries of the selected IC features are small. It is impractical to sputter metal, either aluminum or copper, to fill small diameter vias, since the gap filling capability is poor. To deposit copper in the lines and interconnects of an IC interlevel dielectric, a chemical vapor deposition (CVD) technique has been developed. In the CVD process, copper is combined with a ligand, or organic compound, to make the copper volatile. Copper then becomes an element in a compound that is vaporized into a gas. Several copper gas compounds are available and one includes, for example the liquid complex copper+1 hfac,TMVS (hfac being an abbreviation for the hexafluoro acetylacetonate anion and TMVS being an abbreviation for trimethylvinylsilane) with argon as the carrier gas. Selected surfaces of an integrated circuit, such as diffusion barrier material, are exposed to the copper gas in an elevated temperature environment. When the copper gas compound decomposes, copper is left behind on the selected surface.
FIG. 1 is a side cross-sectional view, of a typical CVD processing chamber 10, such as the TxZ Chamber made by Applied Materials, Inc. of Santa Clara, Calif. Chamber 10 includes a chamber body 20 that defines a cavity. A pedestal 30 is disposed in the cavity of the chamber body 20 and supports a substrate 40 on its upper surface 45 for processing. A gas supply unit (not shown) provides precursor gases to the chamber 10 which react with the substrate 40. A vacuum pump 50 communicates with a pumping channel 60 formed in the chamber 10 to evacuate the gases from the chamber 10. The vacuum pump 50 and the pumping channel 60 are selectively isolated by a valve disposed between the pumping channel 60 and the vacuum pump 50.
There are problems associated with the use of copper in IC processing. One problem with the use of copper is that copper diffuses into silicon dioxide, silicon and other dielectric materials. Therefore, barrier layers become increasingly important to prevent copper from diffusing into the dielectric and compromising the integrity of the device. Barrier layers for copper applications are available for inter-dielectric applications. The use of a thin silicon nitride (SiN) layer on the interlayer dielectric will effectively inhibit interlayer diffusion. Within the same dielectric layer it is difficult to provide an effective barrier to prevent leakage between lines. Several technologies are presently under investigation which add a barrier liner to the via sidewall separating the copper metal from the interlayer dielectric. Common physical vapor deposition (PVD) technologies are limited in high aspect and re-entrant structures due to the directional nature of their deposition. The barrier thickness will depend directly upon the structure architecture with the barrier becoming thinner on the sidewall near the structure bottom. Under overhangs on re-entrant structures the barrier thickness, and therefore the barrier integrity, will be compromised.
In contrast, CVD deposited films are by their nature conformal in re-entrant structures. Further, CVD deposited films maintain a high degree of conformity to the structure""s lower interface. Silicon nitride (SixNy) and titanium nitride (TiN) prepared by decomposition of an organic material are common semiconductor manufacturing materials which display the described conformal performance. Both materials are perceived as being good barriers to Cu interdiffusion, but are considered unattractive due to their high resistivity. The high resistive nature of the material would detrimentally effect the via resistance performance which must be maintained as low as possible. The conduction characteristics of the semiconductor regions are also important considerations in the design of a transistors and the fabrication process is carefully controlled to produce semiconductor regions in accordance with the design. Elements of copper migrating into these semiconductor regions can dramatically alter the conduction characteristics of associated transistors.
Various means have been suggested to deal with the problem of copper diffusion into integrated circuit material. Several materials, especially metallic ones like titanium nitride (TiN) have been used as barriers to prevent the copper diffusion process. A barrier layer of TiN can be deposited by either CVD or PVD, but CVD enjoys the advantage of more easily forming conformal layers in a hole that is relatively deep and narrow.
One CVD process for conformally coating TiN in a narrow hole is the TDMAT process. Referring again to FIG. 1, in the TDMAT process, a precursor gas of tetrakis-dimethylamido-titanium, Ti(N(CH4)2)4, is injected into the chamber 10 through the showerhead 150 at a pressure from about 1 to about 9 Torr while the pedestal 30 holds the substrate 40 at an elevated temperature in a range of about 360xc2x0 C. to about 450xc2x0 C. Thereby, a conductive and conformal TiN layer is deposited on the substrate 40 in a CVD process. Because a TiN layer initially formed by the TDMAT process includes an excessive amount of carbon in the form of included polymers which can degrade its conductivity, the TDMAT deposition is usually followed by a second step wherein the deposited TiN layer is treated with a plasma. In the plasma step, the TDMAT gas in the chamber is replaced by a gas mixture of H2 and N2 in about a 50:50 ratio at a pressure of 0.5 to 10 Torr, and the power supply 170 is switched on to create electric fields between the showerhead 150 and the pedestal 30 sufficient to discharge the H2:N2 gas to form a plasma. The hydrogen and nitrogen species in the plasma reduce the carbonaceous polymer to volatile byproducts which are exhausted from the system and the quality of the TiN film is improved.
Titanium nitride is used primarily to function as a barrier layer. Its value as an adhesion or glue layer for a subsequently deposited CVD Cu layer has been hampered, due mainly to the presence of O2 and other impurities in the surface of the TiN barrier layer at the time of deposition of CVD copper. Reactive CVD copper requires bonding sites for nucleation at the surface of the TiN layer. Impurities, like O2 on the surface of the TiN layer do not allow proper bonding and the copper will not adhere and grow upon the TiN surface. Oxides are present on the surface of a TiN barrier layer even when the TiN layer and Cu layer are both deposited in the same vacuum chamber, as they are in the TxZ Chamber shown in FIG. 1. Various attempts have been made to overcome the problem of impurities in TiN layers that cause poor nucleation of reactive CVD copper. For example, a seed or flash coat of copper over the TiN surface literally covers up the problem and creates a thin surface of identical material for the subsequent deposition of copper. However, a flash coat of any material is typically performed in a separate, PVD chamber requiring a cost and time consuming transfer of the substrate to and from the CVD chamber where the TiN layer and copper layers are deposited.
Accordingly, there is a need in the manufacture of semiconductor devices, for a more effective method of nucleating copper, particularly on TiN barrier films on substrates. There is a further need to employ a method of improving the adhesion of CVD copper to a diffusion barrier material surface without the need of transferring the substrate to another chamber between CVD deposition of the TiN and Cu layers. There is yet a further need for a method of preparing a diffusion barrier surface, in advance of CVD copper deposition, to improve the adhesion of copper to the diffusion barrier surface. There is yet a further need for a process to promote the nucleation of copper onto TiN layers on semiconductor wafers using a precursor gas to avoid or to overcome the presence of O2 on the surface of the TiN layer at the time of the CVD copper deposition.
The present invention generally provides a method of improving the adhesion of a CVD copper layer to a barrier layer on a substrate. In one aspect of the invention, an amorphous layer of silicon (Si) is deposited in a plasma enhanced CVD chamber after the deposition of the TiN barrier layer and prior to the Cu deposition and nucleation steps to provide good adherence of the Cu to the substrate. After deposition of the TiN layer, a plasma is struck in the CVD chamber over the substrate using a silicon source gas, such as silane, and an inert gas, such as argon (Ar). A thin amorphous Si layer is thereby formed over the TiN layer on the substrate. Upon deposition of the Cu layer, the Cu diffuses into the amorphous Si layer to improve nucleation of the Cu. Good adhesion to the substrate is achieved in this manner at the Si/Cu interface.
In another aspect of the invention, a TiSiN layer is deposited by introducing silane gas and a titanium source gas, such as TDMAT, during deposition of the TiN barrier layer. A Cu layer is then deposited on the TiSiN layer. This Cu layer can be, for example, a CVD Cu bulk deposition layer, a CVD Cu seed layer for electroplating, or a Cu electroplating layer.